Known in the art is a writing implement provided with a housing, a pen tip attached to a front end of the housing, an ink tank formed inside the housing, an ink guide member guiding ink in the ink tank to the pen tip, and an air passage formed between the ink tank and the pen tip in the housing and having one end connected to an end of the ink tank at the pen tip side and another end connected to the outside of the housing.
In this writing implement, air flows into the ink tank for the amount of consumed ink. However, if the air in this ink tank expands due to a change in the ambient air pressure or temperature etc., the ink in the ink tank is liable to be pushed out by the air from the ink tank and to flow out through the pen tip or air passage to the outside of the writing implement.
Thus, there is known a writing implement dividing the inside of the ink tank by partition walls into a plurality of ink chambers arranged in the longitudinal direction, arranging an ink guide member so as to extend inside a through hole formed at the center of the partition walls, using the ink guide member to guide the ink in these ink chambers from the ink chambers at the pen tip side successively to the pen tip, using an ink film formed in a ring-shaped clearance between the outer circumferential surface of ink guide member and the inner circumferential surface of the through hole to hold the ink in the ink chambers, and making air flow into the ink chambers through this clearance (see Japanese Patent No. 3436728, FIG. 1 etc.). By doing this, roughly speaking, the air flowing into the ink tank is separated from the ink and is communicated through the air passage to the outside of the housing. Therefore, even if the air in the ink tank expands, the ink in the ink tank is kept from being pushed out by the air to the outside of the ink tank.
In the writing implement described in Japanese Patent No. 3436728, the ink guide member is comprised of a bundle of fibers formed by bundling together a large number of fibers and is held at its two ends at the housing. In this case, the ink guide member itself is low in strength, so in practice it is extremely difficult to form a uniform clearance between the outer circumferential surface of the ink guide member and the inner circumferential surface of the through hole. Therefore, in practice, the clearance between the outer circumferential surface of the ink guide member and the inner circumferential surface of the through hole becomes uneven in the circumferential direction. That is, there are portions where the clearance is large and portions where it is small. However, while explained in detail later, at the portions where the clearance is large, the ink holding force of the ink film becomes smaller and therefore the ink is liable not to be able to be reliably held in the ink chambers.
In this case, if providing a cutaway part in the outer circumferential surface of the partition wall and using the ink film formed at this cutaway part to hold the ink in the ink chamber and making air flow through this cutaway part to the inside of the ink chamber, it is considered that this problem can be solved (see Japanese Patent No. 3436728, FIG. 19 to FIG. 21 and Japanese Patent Publication (A) No. 62-220400, FIG. 1).
In this regard, when providing a cutaway part in the outer circumferential surface of the partition wall, in the state with the writing implement tilted with respect to the vertical line, the position of the cutaway part, that is, the ink film, with respect to the longitudinal axis of the writing implement will change at each instance. That is, there will be cases where the ink film is positioned above the longitudinal axis of the writing implement and cases where it is positioned below it.
However, the behavior of the air passing through the ink film and flowing into the ink chambers can change in accordance with the position of the ink film with respect to the longitudinal axis of the writing implement. That is, the behavior of the air flowing into the ink chambers is liable to differ between the case where the ink film is positioned above the longitudinal axis of the writing implement and the case where it is positioned below it. This means that the behavior of the air in the ink tank is unstable.
Note that in the writing implement of Japanese Patent Publication (A) No. 62-220400, a ring-shaped clearance (33) is formed between the outer circumferential surface of each partition wall and the inner circumferential surface of the ink tank, but since the cutaway part (36) is formed, no ring-shaped ink film is formed.